Rainfall Distribution:
The performance of the summer monsoon over India is evaluated by
the areal averages of seasonal rainfall anomalies over different meteorological
sub-divisons and districts. The India Meteorological Department (IMD)
classifies the rainfall as excess, normal, deficient and scanty if the
areal mean anomaly is: greater than +20%, +19 to -19%, -20 to -59% and
-60 to -99% respectively.
At the end of the monsoon season, 32 out of 35 meteorological sub-divisions received excess to normal rainfall. The rainfall was deficient only in three sub-divisions, viz., Marathwada (-32%), Telangana (-27%) and Vidarbha (-22%). These 32 sub-divisions cover 92% of the area of the country. Districtwise, during this monsoon season, 80% districts in the country received normal to excess rainfall.
For the country as a whole, this year's monsoon has been normal, with the all-India mean rainfall being 102% of its long period average rainfall. There was some lull in the rainfall activity during the month of June, but from July onwards, the cumulative rainfall remained consistently near or above normal. The subdivisions of Marathawada, Vidarbha, Telangana and Coastal Andhra Pradesh received deficient rainfall all through the season. However, deficiency of Coastal Andhra Pradesh was wiped out by a cyclonic storm at the fag end of the monsoon season.
Cyclonic Storms and Depressions:
On an average, 4 to 6 monsoon depressions form in the Bay of Bengal during the monsoon season producing copious rainfall over large areas. In 1997, for the first time in the last five years, one cyclonic storm, four deep depressions and one depression formed during this season in the Bay of Bengal. The deep depressions formed one each in June and July and two in August, while the only depression formed in August. These weather systems, after crossing the Orissa coast, moved in a north-westerly direction and caused widespread rainfall over central and northwest India. Some loss of life and property were reported from Himachal Pradesh, south Rajasthan, Gujarat, Maharashtra and Bihar due to heavy rains and floods. There had been no prolonged breaks in monsoon rainfall over the country in this year.
A cyclonic storm formed in the Bay of Bengal on 24 September and caused very heavy rainfall over Coastal Andhra Pradesh, Orissa and Gangetic West Bengal. The system glided along the east coast and finally crossed Bangaladesh coast in the early hours of 27 September.
Withdrawal:
The withdrawal commenced from the extreme north-western parts of
the country on 6 September against the normal date of 1 September.
By 30 September, south-west monsoon has withdrawn from north-west India
and parts of central India. It has withdraw from the remaining parts
of the country in early October, proceeding in a normal manner.
SHIVA Discussion:
In 1997, the tropical circulation has been dominated by the effects of the major El Nino which grew rapidly during the spring so that by the monsoon season, SST anomalies in the East Pacific were at record warm levels. Past records would suggest that all-India summer monsoon rainfall (AISMR) could be well below normal this year due to the influence of El Nino. In fact, the April 500hPa ridge along 75E was in a record southern position, suggesting extreme drought.
Onset of the Indian Monsoon was over one week late. This appeared to be associated with the Madden-Julian Oscillation (MJO) which has been particularly strong this spring. The active phase of the MJO crossed the Indian Ocean in mid May, spawning an intense Bay of Bengal cyclone. At the normal onset date of 1 June, the suppressed phase of the MJO was in place and appears to have delayed the onset.
AISMR was close to normal but there were unusual regional patterns in the rainfall distribution. Central and southern India were up to 25% drier than normal whereas Northwest India was up to 30% wetter than normal, primarily associated with a series of intense monsoon depressions that moved along the monsoon trough in August and deposited over 400mm of rain in 24 hrs in the vicinity of Mumbai (Bombay).
The reasons why the monsoon was not weakened substantially by the
El Nino are very important to be pursued, and have profound implications
for seasonal forecasting. The precipitation and OLR anomalies suggest a
very pronounced eastward shift in the equatorial convection with marked
supressed conditions existing over Indonesia and extending westwards into
the equatorial Indian Ocean. Also the SST patterns in the NW tropical Pacific
contributed to the very active typhoon season. Both factors pointed to
the likelihood for a more northerly position of the TCZ this year.
Fires and Smoke in Southeast Asia
El Nino .....